This invention relates to highly pigmented liquid compositions which are converted to solid coatings upon exposure to ultraviolet light.
This invention further relates to a method for preparing solid, pigmented coatings on a variety of substrates using liquid compositions containing specified epoxides, liquid carboxylic acid anhydrides, catalysts, and up to 50% by volume of one or more pigments. The compositions are solvent free, in that substantially no loss in weight occurs during conversion of the liquid composition to a solid coating.
Clear or lightly pigmented solid coatings exhibiting acceptable levels of hardness and solvent resistance are obtained by polymerizing epoxide-containing compositions in the presence of the Lewis acids produced when certain diazonium salts, i.e. the fluoroborates, are irradiated with ultraviolet light, as disclosed in U.S. Pat. No. 3,205,157. Up until now it has not been possible to utilize this technology to prepare commercially useful coatings containing sufficient pigment to obtain the high level of hiding power desired for many end-use applications.
If they are applied in sufficient thickness to obtain the desired level of hiding power, highly pigmented liquid compositions containing one or more epoxide monomers or oligomers as the sole or substantially sole reactive component of the liquid phase usually cannot be completely polymerized for form solid materials during an exposure to ultraviolet light in the presence of diazonium salts of Lewis acids as the polymerization catalyst. While surface of the coating may be dry to the touch following irradiation, the material below the surface is often either uncured or insufficiently cured to obtain a significant degree of adhesion to the substrate. In some instances the coating is readily separated from the substrate.
It is known that epoxides can be polymerized at elevated temperatures in the presence of various acidic and basic curing agents, including anhydrides of dicarboxylic acids. A relatively long heating period of several hours is usually required to obtain a solid polymer from compositions having a sufficiently long "pot life" or period of processability at ambient temperatures. This technology is therefore not applicable to the preparation of coatings, which usually must be converted to a dry, non-transferable material within several minutes or less after being applied to the substrate.
It has now been found that by replacing a portion of the epoxide component with a liquid comprising one or more anhydrides of di- or polycarboxylic acids, highly pigmented compositions are converted to hard, solid coatings following exposure times of ten seconds or less to ultraviolet light in the presence of certain aryl diazonium compounds. The coatings can be applied in a thickness sufficient to obtain the desired level of hiding power.
One objective of this invention is to provide pigmented liquid compositions containing epoxides and carboxylic acid anhydrides that can be applied to a variety of substrates and subsequently converted with substantially no loss in weight to a hard, durable coating of acceptable covering or hiding power following a short exposure to ultraviolet light.
A second objective of this invention is to provide a method for rapidly preparing hard, glossy, highly pigmented coatings from liquid epoxide resins.